When D2D communication is enabled in a wireless communication system, it is required for each D2D-capable UE to find its neighbors, i.e. other D2D-capable UEs in close physical proximity, with whom it can communicate directly. The wireless communication system may be e.g. a Wireless Local Area Network, WLAN, or a cellular system such as Wideband Code Division Multiple Access, W-CDMA, or Long Term Evolution, LTE.
There are prior art methods for a D2D-capable UE to find its neighbors. In one type of method, as described in the published patent applications US20130170470 and WO2012170794, a so called decentralized beaconing mechanism is described where a first D2D-capable UE sends a beacon signal for other D2D-capable UEs to respond to. Based on the received signal strength of signals sent from the other D2D-capable UEs in response to the sent beacon signal, the first D2D-capable UE may calculate which of the other UEs that is the best for D2D communication.
A problem with such methods is the time it takes to discover the D2D-capable UEs. If this discovery process would take too long, it may be useless, since no time is left for data transmission, and the neighboring UEs and the first UE may have moved since the discovery process started. Also, good stopping criteria for stopping such a discovery process are difficult to effectively define, especially since the surrounding environment is unknown and in an area around the first UE there may be everything from only a few UEs to a large number of UEs.
Another problem with such a method is the high consumption of battery power for the UEs. This is especially a problem if the amount of possible neighboring UEs is large. Further, an exchanging protocol has to be defined between the D2D-enabled UEs to make them react on each other's beaconing signals. This also implies that more network signaling will take place in the network. There is also an increased risk of a security attack if a UE that fakes its ID sends and receives beaconing signals to from other D2D-enabled UEs.
Consequently, there is a need for an improved mechanism for a first D2D-enabled UE to discover neighboring D2D-enabled UEs.